Figure 6. Model of water system around Lake Okeechobee with mathematical terms
and average flows and storage of water. Flows are in acre-feet per
year and storage is in acre-feet.

AGRICULTURAL
CANALS

generally about 1% with a maximum of 10% in the 1951-60 period. This

difference may be attributable to unmonitored backpumpage.

Figure 8 is a comparison of lake volumes, surface inflows and

outlows and rainfall for the periods 1951-60 and 1965-74. Average

monthly lake volume for the earlier period ranged from 3,160,000 acre-

feet at the end of May to 3,920,000 acre-feet at the end of October,

whereas the 1965-74 period had extremes in the same months but a lower

minimum (2,910,000 acre-feet). River basin rainfall averaged 49.16

inches per year during 1951-60 and 47.66 inches per year during the

later period. Similarly, the rain on the lake was slightly less during

the decade 1965-74. In comparing rainfall-runoff relationships, peak

inflow to the lake occurred earlier in the rainy season and the dry

season runoff was lower during the period 1965-74. Summer inflows were

30% of the annual total during 1951-60 and 42% of the total inflow

during 1965-74.

A set of cumulative rainfall-runoff graphs of the Kissimmee River

in Fig. 9 shows 36,000 acre-feet of discharge per inch of rainfall

occurred during 1951-60 and 22,000 acre-feet of runoff per inch of rain

occurred during 1965-74.

The simulation model in Fig. 10 was used to examine the effects

of differences in timing of influent runoff on lake stages. Values

used in the simulation of normal conditions are presented in Table 1;

storage were expressed in billion cubic meters and flows were expressed

in billion cubic meters per year. Dimensions of sinusoidal parameters

used in various simulations are presented in Table 2. Simulations shown

in Fig. 11 are two-year hydrographs of lake volume under average,

drought and flood conditions. Simulations that resulted from

Figure 7. Results of Lake Okeechobee water budget simulations
for three periods: August 1973 July 1974, 1951 -
1960, and 1965 1974. Simulations are compared
with empirical data.

*I HS I
alt

19

4000-

3500-

3000-

1951-1960

.0

0
o

-SIMULATED
o MEASURED

J I F MIA IM 'J J I A S ONID

Figure 8. Comparison of certain aspects of the water system
of Lake Okeechobee during ten years prior to
channelization (1951-60) and after channelization.

C

ta
*1

0
'p

-

a.
C

C
S
a.
S

I-

U
C

r-
(0

IL

I I I I
100 200 300 400
CUMULATIVE RAINFALL,

I I
100 200
CUMULATIVE

I I
300 400
RAINFALL,

0

1951- 60

*
*
*
*

*
*0

820-
x

z 10-

o 5

3 C

20

I7i
*L

15-

5
-I

< f

600

500
Inches

I

1964-73

*

*

*
*
0
eS

500
Inches

600

Figure 10. Water system model used to test effects of water
management practices on lake stage. Flows are in
billion cubic meters per year and storage are in
billion cubic meters. Regulation and backpumpage
flows are maximum instantaneous values which are
switched by various parameters in the model.

29

I

nation
vels

TABLE 1. Table of Average Values Used in Simulation
of Water System Model Shown in Fig. 10.

Note

Description

Parameter

klR

k2K

k3W

k4Q1

k5QI

k6B

k7R

k8W

water on county area

Value, billion m

2.29/yr

2.39/yr

2.00/yr

10.00/yr

0.36/yr

9.11/yr

20.60/yr

17.50/yr

2.59/yr

4.77

1.56

1 mean rainfall on lake

2 average flow from tributaries
into lake

3 normal evaporation from lake

4 maximum regulation flow out
St. Lucie and Caloosahatchee

5 average flow out of canals at
southern end of lake

6 maximum backpumping rates

7 mean rainfall on Dade, Broward,
and Palm Beach Counties

8 mean evapotranspiration from
counties.

9 other water losses from counties

.0 water in lake at 14.75' MSL

k9Q2

Q1

Q2

- --------- ------ ---------- ------ ---------

TABLE 1. (continued)

1. United States Geological Survey (USGS) by Hartwell reported
average rainfall on Lake Okeechobee to be 2.29 billion cubic
meters per year.

2. In USGS report by Hartwell he states tributary inflow to
Lake Okeechobee amounts to 2.39 billion cubic meters per year.

3. In Hartwell's USGS report he states that average evaporation
from Lake Okeechobee is 2.00 billion cubic meters per year.

4. In USGS Surface Water Records for Florida extreme flows are
given for the regulation canals which total 10.0 billion cubic
meters per year.

5. Hartwell reports long-term average flow of water out of the
Miami, Hillsboro, North New River, and West Palm Beach Canals
combined average (1940-63) 360,000 cubic meters per year.

6. USGS Surface Water Records for Florida report a maximum
short term backpumping rate of 9.11 cubic meters per year.

7. According to Parker and others (1955) average rainfall on
SE Florida is 53 inches per year. If this is prorated over 2
the area of Dade, Broward, and Palm Beach counties (5907 mile )
this equals 20.6 billion cubic meters per year.

8. Parker and others (1955) reported average evapotranspiration
of 45 inches annually in SE Florida which converts to 17.5
billion cubic meters when prorated over the entire area of the
three counties.

9. Hartwell reported other runoff from the counties to be about
2.59 billion cubic meters per year.

10. From U. S. Army Corps of Engineers stage-rating curve for the
lake the volume of water at 14.75 feet above MSL is 4.77 cubic
meters.

11. Prorating four inches of water over the county area this totals
1.56 billion cubic meters.

the same time because the total inflow reaches the regulatory discharge

capacity.

Another analysis of the effects of channelization as it affects

timing of inflow and lake storage is given in Tables 3, 4, and 5.

Table 3 is long term (1931-65) rainfall and evaporation on the lake.

Table 4 lists average monthly surface inflow during 1951-60 (A),

normalized monthly surface inflow during 1965-74 (B), net atmospheric

water (C), total inflow to the lake for 1951-60 (A+C), and total inflow

to the lake to the lake for 1965-74 (B+C), In Table 5 monthly discharge

is calculated to equal the amounts of water needed to released for

closest maintenance of regulation regime.

Figure 11.

Results of simulating water model shown in Fig. 10.
Simulations of lake storage in which rainfall and
runoff were synchronous is shown by solid line.
Lake storage affected by runoff lagging three months
after rainfall is shown by broken lines. These are
two year hydrographs for normal (a), drought (b),
and flood (c) conditions.

(a)

Normal

--^ 1%

.4

Table 4 shows four months of net loss of lake water because of

changes in timing of runoff during 1965-74 and no months of net water

loss during the earlier period. Table 5 shows more water (248,000 acre-

feet) must be released in the summer months of the 1965-74 period because

of higher total summer runoff. Table shows lower stages at the end of

the year in the 1965-74 period. Agricultural water requirements would

affect this analysis by making ending storage even lower for the 1965-74

period.

Phosphorus Systems in Lake Okeechobee

A model of the phosphorus system in the lake was developed to

investigate response times and long term changes in phosphorus levels as

they are affected by various nutrient input rates and sediment-water

exchange rates. The model of phosphorus in Lake Okeechobee includes

allochthonous sources and sinks, water-marsh exchanges and water sediment

2. The author made biomass clippings of various marsh com-
munities in Lake Okeechobee near Clewiston during August 1974. A
quarter m2 of marsh was cut, dried at 70C for five days and weighed;
the dry weights were multiplied by the factor 0.4 to convert to
grams carbon. The results were: bullrush-680 g C/m2, cattail-380
g C/m2, spikerush-150 g C/m2, peppergrass-90 g C/m2, pond lilly-100
g C/m2 (estimated). The weights were multiplied by the areas of these
communities given by Ager (1974) to get a total of 65.2 X 109 g C
in marsh plants. This equals 37.6 g C/m2 when prorated over the lake
area of 1.736 billion m2. Assuming a C:P ratio of 100:1 in these
plants, the average phosphorus standing crop is 0.376 g P/m2.
Phosphorus in marsh soils can be estimated by assuming a bulk
density of 1 g dry wt/cm3 which equals 104 g dry wt/m2/cm depth.
Assuming depth of sediments in marsh is 5 cm deep and concentration
is 0.27 x 10-3 g P/g dry wt of sediment (Joyner, 1974). The stand-
ing crop of phosphorus in marsh soils is (5 x 104 x 0.27 x 10-3 =)
13.5 g P/m2 to 5 cm. Prorate this value overmarsh sediment area
(0.276 billion m2) and the whole lake area (1.736 billion m2) which
equals 2.15 g P/m2.
The phosphorus in the marsh plants and soils is 2.53 g P/m2.

3. Total phosphorus stored in sediments in the open water zone
can be estimated from calculation in note 2 and prorating the stand-
ing crop to 5 cm (13.5 g P/m2) over the area of open water zone
dividing by the area of the whole lake. This equals 11.35 g P/m2
of lake.

4. In Table 6 is the estimation of the phosphorus load de-
livered to the lake in 1952 (Odum, 1953). Phosphorus load was
estimated to be 0.102 g P/m2/year.

5. As estimated by Joyner (1974) and correcting for time and
area of the lake this amounts to 0.389 g P/m2/year.

7. Area based loading rates from Lake Apopka was estimated from
Sheffield and Kuhrt (1970) by totaling the Zellwood drainage district
load of 800,000 lbs o-P04 or 0.9 g P/m2/year, Winter Garden area load
of 245 lbs o-P04/day or 0.11 g P/m2/year, and rain influx of about
0.082 g P/m2/year. This totals 1.1 g P/m2/year.

8. Using the model Kirchner and Dillon (1975) based on areal
loading rates of water to a lake, it predicts the retention of about
80% of the phosphorus entering Lake Okeechobee. Twenty percent
leaves which is (0.20 x 0.102 g P/m2/year) 0.0204 g P/m2/year.

12. Average water depth on the marsh zone (8 to 14 foot MSL
contours) is about one meter. The model of water circulation
(Fig. 23) predicts average current velocities of 0.01 feet per se-
cond under high flow conditions; assume velocities are 0.001 feet
per second under average conditions. Water velocities of 0.001
feet per second equal about 1 meter per hour. Assume water exchanges
occur over a length of marsh roughly equal to the periphery of the
lake (129 km). Lake water at 0.054 g P/m3 (Davis and Marshall,
1975) being advected at 1 m/hour over marshes 1 meter deep will
deliver 0.054 g P/m2/hour or 473.04 g P/m2/yr. As lake water meets
the marsh periphery, 473 g P/m2/yr or (129 x 103m x 473 g P/m2/
year) 61 x 106 g P/periphery/year is delivered. Prorating the
delivery over a marsh area of 0.276 x 109 m2 (Ager, 1974) this equals
0.22 g P/m2/year. Prorating over entire lake area of 1.736 x 109
m2 the average flow of phosphorus to the marsh is 0.035 g P/m2/year.

14. According to Kamp-Nielson (1974) sorption of phosphorus into
aerobic lake muds follows the equation Y = -1.38x + 1.88, where: x
is the log of the concentration of pg P-P04/X in overlying waters and
Y is the rate of sorption in mg P-P04/m2/day. At a pH of 8.4 the
Y-intercept of the regression equation is closer to 1.13, although
in the simulation a range of values was used. Total phosphate levels
were corrected to o-P04 levels by a factor of 0.65 from Joyner (1974).
Solution of this equation for Lake Okeechobee when pH is 8.4, Y-
intercept is 1.13, and P04-P is 35 pg/e yields the sorption rate
of .365 gP/m2/year.

15. Sedimentation rate is assumed to the phosphorus equivalent
of the amount of carbon produced (2.1 gC/m2/day) plus the alloch-
thonous flows (0.07 gC/m /day) minus the amount of respiration in
dark bottles measured by Davis and Marshall (1975) minus the amount
consumed by higher animals (see calculations of carbon flows in
metabolism model). This results in a sedimentation rate of

47

1.76 gC/m2/day or 642 gC/m2/year. Assuming the C:P ratio was
100:1 in this material, the amount of phosphorus sedimented
equals 6.42 gP/m2/year. The production and respiration values are
from Davis and Marshall (1975). The level of higher consumption
is estimated from data on fish, zooplankton, and bottom animal
populations by Joyner (1974), Ager (1974), and Davis and Marshall
(1975).

16. Assuming sedimentation and recycle are equal, the recycle
of phosphorus was evaluated at 6.42 gP/m2/year. If this is equiva-
lent to 642 gC/m2/year or 0.18 g02/mm/hr, it places sediment respir-
ation in the range normal for Florida lake bottoms (Tom Ballinger,
pers. comm.).

TABLE 7. Estimated Phosphorus Loads to Lake Okeechobee During 1952*

Tributary Concentration, mg/ Flow, acre-feet Load, xlO gP/yr

Kissimmee 0.007 1,411,000 1.219

Fisheating Creek .031 308,300 1.179

Taylor Creek .057 73,840 .519

Indian Prairie .121 35,400 .529

Total phosphorus load from tributaries is 3.446 x 107 gP/yr or .020
gP/m2/yr. Load from rain averages 0.082 gP/m2/yr (Joyner, 1974).
Total load to lake in 1952 is 0.102 gP/m2/yr.

* Concentrations measured by Odum (1953) in summer of 1952. Flows from
the Kissimmee and Fisheating Creek were from USGS water records,
whereas flows from Taylor Creek and Indian Prairie were long-term
averages from Joyner (1974).

on the levels of phosphorus in the lake. Loading.rates calculated from

studies by Odum (1953), Joyner (1974), Davis and Marshall (1975),

Sheffield and Kuhrt (1970) affect the results of simulations in Fig. 14a,

14b, 14c, and 14d, respectively. The phosphorus loads based on Odum's

study represent more natural conditions, loads based on Joyner's study

and Davis' study represent present levels, and loads based on Sheffield's

work with Lake Apopka represent very high load rates.

Figure 14 shows that the rate of phosphorus build up increases with

load rate. After 25 years water column phosphorus levels show increases

of 12%, 49%, 58%, and 147% with each increasing net load to the lake in

Figs. 14a-14d. This equals rates of increase of water column phosphorus

of 0.6 parts per billion (ppb)/year, 2.6 ppb/year, 3.1 ppb/year, and

7.8 ppb/year, respectively for Figs. 14a-14d. Joyner (1974) measured

an average 47 ppb total phosphorus from 1969-72 and Davis and Marshall

(1975) reported an average 54 ppb total phosphorus from January 1973

to May 1974. Odum (1953) reported total phosphorus levels lower than

any measured during later studies which averaged 13 ppb. The difference

between earliest measurements and latest measurements is 41 ppm rof: water

column phosphorus. The marshes in these simulations remained the same

because of the small rates of exchange between water column phosphorus

and marsh phosphorus.

Figure 15a shows the same simulations as 14c which is the lake under

the influence of 25 years of phosphorus loads measured by Davis. All of

the following simulations in Fig. 15 and Fig. 16 have Davis' loading rates

but have variations of other parts of the model. Figure 15b shows the

Figure 14.

Results of simulating phosphorus model shown in
Figs. 12 and 13 for 25 years of four different
inflow rates. In order of increasing net annual
delivery to the lake were: (a) inflows of 1952
(Odum, 1953); (b) inflows of 1969 (Joyner, 1974);
(c) inflows of 1974 (Davis and Marshall, 1975); and
(d) inflows of area-corrected flows to a hyper-
eutrophic lake, Lake Apopka (Sheffield and Kuhrt, 1970).

a.

w

S

M

10 20

10 20 10 20
YEARS

N

CL
E

20
b

0
IL 15-

.0
z

o

I-

S5-

10 20

effect of phosphorus losses by aerosol formation which equals rainfall

loads. In this simulation water column phosphorus increases 41% in 25

years. Water column phosphorus rises similarly in Fig. 15c when marsh

uptake is increased five times. Figure 15d shows the effect of marshes

dying off and releasing phosphorus to the water column. This accelerates

rises in water column phosphorus to 79% over 25 years; sediment phosphorus

shows similar rise.

Figure 16a shows phosphorus levels from Davis- loading rates.

Figure 16b shows the effect of desorption of sediment phosphorus under

3. Davis and Marshall (1975) reported an average daily primary
production of 2.1 gC/m2/day. Assuming for every 100 grams of carbon
fixed there is 1 gram of phosphorus fixed, and half of the phosphorus
fixed is supplied internally from phosphorus storage in the planktons.
Daily assimilation of phosphorus equals 0.011 gP/m2/day. This needs to
be measured in lab or field experiments.

4. Davis and Marshall (1975) reported respiration was 10% of
gross primary production (GPP) which they estimated to be 2.1 gC
GPP/m2/day. In percent of the GPP is 0.21 gC/m2/day of plankton respi-
ration. If the consumer population which has a standing crop of 18.6
gC/m2 (see note 23), and assuming 90% of the consumer's food require-
ment comes from the water column, this represents a flow of 0.18
gC/m2/day from water column carbon to consumers. The recycle of phos-
phorus from plankton respiration (0.21 gC/m2/day) and consumer activi-
ties (0.18 gC/m2/day) assuming a C:P ratio of 100:1 and a release of
one-half this amount (0.18 + 0.21)(0.01)(0.5) equals 0.002 gP/m2/day.

5. The recycle of phosphorus from sediments depends on the rate
of respiration of organic carbon in the sediments. The respiration of
organic carbon in sediments is assumed to equal the total fixed (2.1
gC/m2/day) plus allochthonous sources (0.07 gC/m2/day) minus respi-
ration of plankton (0.21 gC/m2/day) and minus the amount consumed by
the higher trophic levels (0.20 gC/m2/day). The calculated sediment
respiration (2.1 + 0.07 0.21 0.20 gC/m2/day) equals 1.76 gC/m2/day.
Recycle of phosphorus equals one-half of the phosphorus equivalent of
carbon respiration (0.5 x 0.01 x 1.76 = 0.0088) minus that which is
sorbed into the sediments. Sorption rates were calculated (from phos-
phorus model) to be 0.365 gP/m2/year or 0.001 gP/m2/day. Phosphorus
recycle (0.0088 0.0010 gP/m2/day) equals 0.0078 gP/m2/day. In simu-
lation this value was rounded off to 0.008 gP/m2/day.

8. Respiration in the water column (see note 4) equals 0.39
gC/m2/day; the oxygen equivalent of this is 0.98 g02/m2/day.

9. Respiration in sediments is 1.76 gC/m2/day plus consumption of
sediments (10% of the food requirement of consumers) which is 0.02
gC/m2/day. The oxygen equivalent of this (1.78 gC/m2/day x 2.5 g02/gC)
is 4.45 g02/m2/day.

11. Allochthonous organic carbon inflows were estimated from
Brezonik's (1975) estimation of total organic carbon (TOC) in back-
pumpage water (100 mg/i) times average backpumpage flow (350,000 acre-
feet/year) and from Joyner's (1974) measurements of TOC in other tribu-
taries which average 20 mg TOC/i and flow at 2,000,000 acre-feet/year.
The inflow of carbon is 0.15 g TOC/m2/day. Outflows of carbon (2,000,000
acre-feet/year at 20 mg/k) equals 0.08 g TOC/m2/day. The net organic
carbon delivered to the lake is 0.07 g TOC/m2/day.

12. The rate of sedimentation equals the amount of carbon fixed
(2.1 gC/m2/day) minus carbon respired and consumed in the water column
(0.39 gC/m2/day) plus allochthonous carbon (0.07 gC/m2/day). This equals
0.78 gC/m2/day of net sedimentation. An arbitrary rate of suspension
of carbon was set at 48 gC/m2/day, which represents the replacement of
the standing crop of water carbon once a day. The gross sedimentation
rate is 1.78 gC/m2/day plus 48 gC/m2/day or 49.78 gC/m2/day.

13. The gross rate of suspension of organic carbon was assumed to
replace water storage once a day: 48 gC/m2/day.

14. Organic carbon respired and consumed from the water column (see
note 4) is 0.39 gC/m2/day.

16. Consumption of water column carbon by consumers was estimated
to be 90% of total food consumed (see note 23): 0.18 gC/m2/day.

17. Consumption of sediment carbon by consumers was estimated to be
10% of total food consumed (see note 23): 0.02 gC/m2/day.

18. Assuming consumers are in steady state their production (0.20
gC/m2/day) equals their respiration and mortality (0.20 gC/m2/day).

19. Standing crop of total dissolved phosphorus was estimated by
averaging Joyner's (1974) phosphorus determination of filtered samples:
0.012 g dissolved and colloidal phosphorus/m3 and 0.029 g P(dissolved)/m2
at an average depth of 8 feet.

20. Saturation of 02 in freshwater at 250C and 760 mm of pressure
is 8.4 ppm. At an average depth of 8 feet this (8.4 g/m3 x 2.44m) equals
20.5 g02/m2. The initial condition was set slightly below this.

21. Total organic carbon measurements by Joyner (1974) and Brezonik
(1975) average 20 mg TOC/i. Over a depth of 8 feet this equals 48.8 gC/m2.
The initial condition in simulation was set slightly lower.

22. At a bulk density 1 cm3/g, one m2 of sediment which is 10%
organic matter (Burton et al., 1975) and one centimeter deep would con-
tain 500 gC/m2.

66

23, Consumer biomass was estimated to be 7.0 gC/m2 of fish (Ager,
1974), 1.0 gC/m2 of zooplankton (typical level of zooplankton for
eutrophic lakes; Nordlie, personal communication), and 10.6 gC/m2 of
benthic invertebrates (numbers of invertebrates Joyner, 1974; Davis
and Marshal, 1975). The total is 18.6 gC/m2. Benthic invertebrate net
production is 73.6 x 109 gC/year, zooplankton net production is 30.9
x 109 gC/year, fish net production is 17.1 x 109 gC/year. The total
net production was calculated to be 0.20 gC/m2/day. The net production
of zooplankton and benthic fauna was estimated from Prosser and Brown
(1961), Hall (1964), and Waters (1969). The net production of fish was
estimated from Gerking (1962), using standing stocks from Ager (1974).
The biomass of benthic fauna was calculated by estimating individual
weights per organism and multiplying them by numbers of organisms esti-
mated by Joyner (1974) and Davis and Marshall (1975).

Figure 19.

Simulation of metabolism model shown in Fig. 18 for five
days of average conditions. Shown are daily changes in
oxygen, phosphorus, sunlight, carbon, and consumers.

Figure 24. Results of simulation of spatial model after five
days of high flow from tributaries and pumps.
Arrows on the channels (lines) show direction of
flow and numbers are flow rates in feet per second.

Scale in

Feet

50
THOUSAND

Flows measured in feet per second.

Sr
i

k I'"

79

N

__ _~__ ~_~~~_~_~_

DISCUSSION

Simulations of models suggest that Lake Okeechobee has changing

water regimes and nutrient inflows due to development in watersheds

around the lake. Further development in the basin could compound

problems in an already eutrophic lake.

Channelization and Water Levels

Channelization of tributaries to Lake Okeechobee has accelerated

runoff so that rainy season flow accounts for greater percentages of

total annual inflow than under prechannelized conditions (Fig. 8).

Although flooding potential is less upstream in these channelized basins,

potential flooding downstream is increased. Moderately high rainfall in

1969 caused record flow rates (21,700 cfs) in the Kissimmee River channel

which seriously damaged the S-65E control structure; hurricane rains will

help determine the efficacy of this flood control system in the future.

Changes in timing of inflow to Lake Okeechobee causes lake stages

to rise more quickly and sooner during the rainy season. In order to

maintain safe storage capacities in the lake some of the inflow is dis-

charged through the St. Lucie and Caloosahatchee canals. Generally,

high rates of inflow require high rates of outflow through regulation

to control lake stages. Consequently, accelerated runoff in channelized

tributaries may increase runoff and flood stress in the regulation canals

SYSTEMS MODELS FOR UNDERSTANDING EUTROPHICATION IN LAKE OKEECHOBEE BY TIMOTHY LAWRENCE GAYLE A THESIS PRESENTED TO THE GRADUATE COUNCIL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 1975

PAGE 2

ACKNOWLEDGEMENTS Appreciation is expressed to Dr. H. T. Odum, my committee chairman, for unique guidance and inspiration. Many contributions were appreciated from members of my committee: Dr. J. L. Fox and Dr. F. G. Nordlie. Work was sponsored by contract between the Florida Department of Pollution Control and the Florida State Division of Planning and the Center for Wetlands, University of Florida as a part of a state project on Eutrophication of Lake Okeechobee. Principal investigators were H. T. Odum and F. G. Nordlie; state project coordinators were Curry Hutchinson and Dale Walker. Thomas Fontaine and Anne Meylan collaborated in project assignments. Data and advice were provided by F. E. Davis and M. E. Marshall of the Central and Southern Florida Flood Control District. Dr. W. Huber helped with the spatial model. ii

TABLE 1 2 3 4 LIST OF TABLES Table of Average Values Used in Simulation of Water System Model Shown in Fig. 10 Median Values and Maximum and Minimum Instaneous Values of Line Wave Function Used in Simulation of Water Model in Fig. 10 Median Rainfall, Normal Evaporation, and Water Supply on Lake Okeechobee Normalized Tributary Inflow, Rainfall, and Evaporation to Lake Okeechobee 5 Discharges and Regulation of Stages in Lake Okeechobee as it is affected by Different Runoff Patterns 6 7 8 9 10 11 12 13 Sources, Storages, and Rates for the Phosphorus Model of Lake Okeechobee (Figs. 12 and 13) Estimated Phosphorus Loads to Lake Okeechobee During 1952 Values Used in Simulating the Model of Metabolic Activities in Figs. 17 and 18 A Water Budget of Lake Okeechobee for October 1950 to September 1960 A Water Budget of Lake Okeechobee for July 1973 to July 1974 A Water Budget of Lake Okeechobee for October 1964 to September 1974 List of Biological Events in Lake Okeechobee Data Used in Running Spatial Model v Page 30 32 37 38 39 44 48 62 92 93 94 107 110

PAGE 6

Figure 1 2 3 4 5 6 7 8 9 10 11 12 13 LIST OF FIGURES Overall model of ecosystem in Lake Okeechobee Simplified model of ecosystem in Lake Okeechobee Location and topographic map of lake Stage-rating curve for lake Description of Energese symbols Model of long-term average water relationships in Lake Okeechobee Water budget simulations of Lake Okeechobee for 1951-60 and 1965-74 Comparison of the water system around the lake for the periods 1951-60 and 1965-74 Cumulative rainfall and runoff for the Kissimmee River, 1951-60 and 1965-73 Model of water system used to investigate effects of changes in basin characteristics Simulations of water levels in lake under drought, flood, and normal conditions with accelerated runoff and normal runoff Model of phosphorus system in Lake Okeechobee with mathematical terms Evaluated model of phosphorus system in lake 14 Simulation of phosphorus model for various loading rates 15 Simulation of phosphorus model for aerosol loss, marsh growth, and marsh die-off vi Page 3 5 8 10 14 20 23 25 27 29 35 41 43 51 54

PAGE 7

Figure 16 LIST OF FIGURES -continued Simulation of phosphorus model for various sedimentwater exchange conditions 17 Model of the metabolic system in lake with mathematical terms 18 Evaluated model of the metabolic system in lake 19 Simulation of metabolic system model for normal conditions 20 Simulation of metabolic system model for low diffusion 21 Simulation of metabolic system model for high phosphorus inflow 22 Simulation of metabolic system model for high organic carbon inflow 23 24 25 26 27 Energese model and equations of water movement for model of spatial characteristics Results of simulating spatial model Analog diagram, differential equations, and scaling variables for water system model Dynamo program for phosphorus model Analog diagram, differential equations, and scaling variables for metabolic system model vii Page 56 59 61 68 70 72 75 77 79 96 100 102

PAGE 8

Abstract of Thesis Presented to the Graduate Council of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science SYSTEMS MODELS FOR UNDERSTANDING EUTROPHICATION IN LAKE OKEECHOBEE ny Timothy Lawrence Gayle June, 1975 Chairman: Howard T. Odum Hajor Department: Environmental Engineering Sciences Systems models were developed to investigate ecological changes and eutrophication trends in Lake Okeechobee, Florida. An energy flow model was used to organize data and concepts on the lake as a system. Simplified models of water budget, phosphorus budget, metabolism, and water currents were simulated to estimate the effects of changes in river flow, water levels, and nutrient loads. Simulations of water budgets from 1951-1974 predicted lake stage within 10%. River basin channelization accelerated runoff into the lake with less water remaining on the land for use and discharge during dry season. Simulations indicated channelization causes an additional 200,000 acre-feet per year to reach the lake during summer months and be discharged to the sea as part of hurricane protection. Present phosphorus loading rates to the lake from rain and tributaries were 0.3 to 0.4 g P/m2/yr which compared with I g P/m2/yr in Lake Apopka. Nearly 90% of this phosphorus load was unaccounted for in outflow and could be trapped by sediment. The turnover time of viii

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phosphorus was about 60 days in the lake and the doubling time may be as short as 15 years under present loading conditions. Based on several diurnal oxygen measurements primary production may be about 2.1 g C/m2/day; this indicates the lake is moderately eutrophic. Average net organic load is estimated to be 0.07 g C/m2/day, which is sufficient to reduce oxygen levels to 85-90% saturation. Simulations indicate that greater organic matter inflows resulting from intensive backpumping or destruction of the marsh can considerably reduce oxygen levels. Raising the lake stage regulation level two feet will drastically reduce the extent of seasonal marshes in the lake that now support wildlife, especially wading bird populations. Simulations indicated doubling phosphorus loads caused slight increases in shortterm production. Simulation of a spatial model indicated water currents circulated in a clockwise direction and rapidly distributed nutrients horizontally in the limnetic zone. ix I

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INTRODUCTION Lake Okeechobee serves south Florida as a valuable water supply, recreational resource, and wildlife area. This important asset, however, may be changing with eutrophication. In this study systems models were used to investigate the effects of eutrophication and other ecological changes on the lake. The model in Fig. 1 represents an overview of ecological interactions within the lake. A simplified model is given in Fig. 2 from which three submodels were evaluated and simulated. Models were used to consider questions on the hydrology, nutrient budget, organic metabolism, and circulation of the lake. The term eutrophy generally applies to waters which are productive and nutrient rich, and the degree of eutrophy depends on the quantities of energy available to the productive process. Moderate production and eutrophy are characterized by high biomass and diverse trophic levels. Extremely high production, or hyper-eutrophy, is characterized by even higher biomass, shorter food chains, less diverse trophic levels, and wide fluctuations in oxygen. Hyper-eutrophic lakes have less game fishing, and decreased water quality for contact sports and water supply. Lake Okeechobee is located in south-central Florida 26041' to 27l3'N and 80031' to 8l007'W, and has over 700 square miles of surface area making it one of the largest lakes in the United States. Geologically, the basin is part of the Pamlico Terrace, a plain formed during the recession of the sea in the late Pleistocene (Joyner, 1974). The lake bottom is variously composed of calcitic muds, marls, and gyttja 1

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Figure 1. Model of Lake Okeechobee ecosystem showing relationships between water, sun, heat, nutrients, organisms, and man. Model drawn by H. T. Odum and F. G. Nordlie based on discussions with staff of Central and Southern Florida Flood Control District and others in 1973.

Figure 2. Simplified model of the ecosystem in Lake Okeechobee which relates water, phosphorus, and metabolism. Simulation models were derived from this simplified model.

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V>

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6 with bottom elevations averaging six feet above sea level and some areas below sea level (see Fig. 3). Because of the shallowness of the lake basin, small changes in water surface elevations are accompanied by large changes in the lake area (see Fig. 4). The climate around Lake Okeechobee is semitropical; ,however.-. this' water body exerts a large influence on the temperature, evaporation, and rainfall patterns on the lake (Riebsame a1., 1974). l!edian rainfall is 45.6 inches per year (Joyner, 1974), 60% of which falls from June through September (Casselman, 1970). Normal evaporation is 55.7 inches per year and is highest in May. Rainfall, evaporation, and runoff are quite variable seasonally and yearly and water storage in the lake reflects this. Drainage basins around the lake have been highly modified for water control, navigation, and agricultural development. The lake is encircled by a levee 80 miles long and 25 feet high, and is located near the 15 foot contour (see Fig. 3). Most rivers flowing into the lake are channelized and their discharge is regulated by flow control structures. Excess water from agricultural lands south of the lake is backpumped into the lake. Lake stage is regulated between 14.0 and 15.5 feet above mean sea level (MSL) and excess lake water is released via two channelized rivers to tidewater. For a description of the lake basin in its pristine state see (1974). Surveys of the lake's water chemistry were conducted by Odum (1953), Parker and others (1955), Holcomb (1968), Duchrow (1970), Joyner (1974), and Davis and Marshall (1975). Davis and Marshall found the lake well mixed vertically, with high levels of calcium, chloride, and dissolved solids. Brooks (1974) attributed the high levels of dissolved solids in

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Figure 3. Location of Lake Okeechobee on peninsular Florida. Map of the lake shows basin topography and sites of major rivers and canals and the Hoover Dike. Redrawn from Brooks (1974).

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C> --o :r D n c .. C ... ----I o 20p00 40pOO Scale in Feet 8 N

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Figure 4. Stage-rating curve for Lake Okeechobee showing relationships between stage, storage, and area. Data is from U.S. Army Corps of Engineers, 1962.

11 the lake to groundwater seepage, high evaporation, and construction of the Hoover Dike. Limnetic phytoplankton and benthic fauna populations were investi-gated by Joyner (1974), and Davis and Marshall (1975). Ager and Kerce (1974) conducted studies on fish feeding habits and fish standing crops in the open-water zone. Sincock and others (1957), Soumela (1958), and Ager and Kerce (1974), conducted surveys of littoral zone vegetation, and fish and wildlife populations. Browder (1974) surveyed wading bird populations in the lake's seasonal marshes. Lake ecosystem models have been developed for Lake George, New York (Park et al.,1972) and Lake Wingra, Wisconsin (Huff al., 1973; MacCormick al., 1972) to mention a few. More generalized lake ecosystem models have been developed by Parker (1968) and Park (1974). A good review of aquatic modeling and the eutrophication process can be found in Middlebrooks and others'{197J). Although little is known about eutrophication in shallow semitropical lakes and few of these lakes have been modeled, general ecosystem principles can be applied to investigate the nature of the eutrophication process in Lake Okeechobee. It is evident from recent studies (Joyner, 1974; Davis and Marshall, 1975; Brezonik and Federico, 1975) that Lake Okeechobee receives an abundant supply of nutrients. According to Brezonik and Federico (1975), loading of nitrogen and phosphorus are near or above "dangerous" levels and cause problems of eutrophy in the lake. In Vollenweider's (1968) study, he concluded that massive growth of blue-green algae is likely if nutrient concentrations exceed 0.01 g P/m 3 and 0.2-0.3 g N/m3 Lake Okeechobee averages 0.05 g P/m3 and 1.4 g N/m3 and can be considered in most aspects a moderately eutrophic lake.

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METHODS Principal methods used were concerned with development, evaluation and simulations of models. Model output was compared with empirical data. Models of water, phosphorus, and metabolic systems in Lake Okee chobee are presented in Figs. 6, 13, and 19, and are depicted in Energese, an energy circuit language developed by H. T. Odum. A description of each symbol and its mathematical equivalent can be found in Odum (197lb and 1972), and in Fig. 5. The algebraic equivalents of these models are presented in Figs. 6, 12, and 18, and sets of differential equations derived from these models were programmed on computers for simultaneous solutions. Water and metabolic systems models were simulated on the Electronic Associates, Incorporated, Miniac analog computer. Analog patching diagrams, scaling factors, and pot settings are given in the Appendices. The model for phosphorus systems was simulated on the University of Florida IBM 370/360 digital computer in Dynamo II, and the program is given in Appendix B. The model used to simulate movement of water within Lake Okeechobee was adapted from the Receiving Water Quantity Model, a submodel of the EPA Storm Water Management Model (1971). In the model water movement is represented by a network of nodes connected by channels which are idealized hydraulic elements. These elements are characterized by parameters 12

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Figure 5. Description of Energese, an energy circuit language used for modeling in this study (Odum, 1971, 1972). (a) External, unlimited energy source to the system. (b) Heat sink as required by the second law of thermodynamics in order to do work. (c) Component of energy or matter storage in which quantity stored is the integral of the inflows and outflows. (d) Interaction in which output is proportional to the product of two input forces. (e) Interaction in which output is some unspecified function of two input forces. (f) Autocatalytic unit which by virtue of feedback mechanisms may enhance its ability to process energy. (g) Force (X) acting in proportion to flow (J).

Figure 5. (continued) (h) Cycling receptor module as in chlorophyll excitation-deactivation cycles "and other anabolism. (i) Autotrophic individual or community which has both anabolic and catabolic processes. (j) Switch used when flows are regulated by on-off signals such as lake regulation or political decisions. (k) Constant gain amplifier used when unlimited source drives low flow without affecting the source. (1) Economic transactor showing opposite flow of money and energy. (m) Two-way workgate which operates according to the gradient and driving force. (n) Box used to lump linear processes. Nutrients can be shown being recycled from respiratory pathways.

17 of length, head, floor elevation, hydraulic radius and frictional coeffi cients. Simultaneous solution of equations of continuity and motion for each element results in changes of stage, velocity and flow in the com ponents of the water system. Hand simulations of water budgets were calculated by combining monthly inflows and outflows to obtain monthly changes in lake storage.

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RESULTS Included in results are the models, data assembled for evaluation, computer simulations, and comparisons with observed data or trends. Models simulated include systems of water, phosphorus, metabolism, and water circulation. Water System of Lake Okeechobee A water model for Lake Okeechobee is presented in Fig. 6 in which mathematical terms and values for stocks and flows are included. Longterm averages for the storages and flows were obtained from U; S. G. S. surface water records and rainfall and evaporation records of the U. S. Army Corps of Engineers. Prior to 1957 many of the surface water flows around Lake Okeechorree were unmonitored. Inspection of the magnitudes of flows in Fig. 6 reveals that rain and river inflows to the lake are roughly equal, as are the outflows of evaporation and runoff. Inflows replace the lake volume about once a year. The short residence time of lake water is reflected in the seasonally variable stages which have ranged from 10.14 to 18.77 feet above mean sea level (MSL). Present water management practices attempt to regulate lake stages between 14.0 feet MSL in the summer and 15.5 feet MSL in the winter, however, average stages are lower than this. Because of the strong seasonality of rainfall and runoff, the lake stages are almost always high in the fall and low in the late spring. Hand calculated simulation water budgets for monthly inflows and outflows are presented in Fig. 7 and are compared with observed data. The percentage difference between simulated and empirical values was 18

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Figure 6. Model of water system around Lake Okeechobee with mathematical terms and average flows and storages of water. Flows are in acre-feet per year and storage is in acre-feet.

21 generally about 1% with a maximum of 10% in the 1951-60 period. This difference may be attributable to unmonitored backpumpage. Figure 8 is a comparison of lake volumes, surface inflows and outlows and rainfall for the periods 1951-60 and 1965-74. Average monthly lake volume for the earlier period ranged from 3,160,000 acrefeet at the end of May to 3,920,000 acre-feet at the end of October, whereas the 1965-74 period had extremes in the same months but a lower minimum (2,910,000 acre-feet). River basin rainfall averaged 49.16 inches per year during 1951-60 and 47.66 inches per year the later period. Similarly, the rain on the lake was slightly less during the decade 1965-74. In comparing rainfall-runoff relationships, peak inflow to the lake occurred earlier in the rainy season and the dry season runoff was lower during the period 1965-74. Summer inflows were 30% of the annual total during 1951-60 and 42% of the total inflow during 1965-74. A set of cumulative rainfall-runoff graphs of the Kissimmee River in Fig. 9 shows 36,000 acre-feet of discharge per inch of rainfall occurred during 1951-60 and 22,000 acre-feet of runoff per inch of rain occurred during 1965-74. The simulation model in Fig. 10 was used to examine the effects of differences in timing of influent runoff on lake stages. Values used in the simulation of normal conditions are presented in Table 1; storages were expressed in billion cubic meters and flows were expressed in billion cubic meters per year. Dimensions of sinusoidal parameters used in various simulations are presented in Table 2. Simulations shown in Fig. 11 are two-year hydrographs of lake volume under drought and flood conditions. Simulations that resulted from

Figure 10. Water system model used to test effects of water management practices on lake stage. Flows are in billion cubic meters per year and storages are in billion cubic meters. Regulation and backpumpage flows are maximum instantaneous values which are switched by various parameters in the model.

Parameter TABLE 1. Table of Average Values Used in Simulation of Water System Model Shown in Fig. 10. 30 Note Description Value, billion m 3 1 mean rainfall on lake 2.29/yr 2 average flow from tributaries 2.39/yr into lake 3 normal evaporation from lake 2.00/yr 4 maximum regulation flow out 10.00/yr St. Lucie and Caloosahatchee 5 average flow out of canals at 0.36/yr southern end of lake 6 maximum backpumping rates 9.ll/yr 7 mean rainfall on Dade, Broward, 20.60/yr and Palm Beach Counties 8 mean evapotranspiration from l7.50/yr counties. 9 other water losses from counties 2.59/yr 10 water in lake at 14.75' MSL 4.77 11 water on county area 1.56

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TABLE 1. (continued) 1. United States Geological Survey (USGS) by Hartwell reported average rainfall on Lake Okeechobee to be 2.29 billion cubic meters per year. 2. In USGS report by Hartwell he states tributary inflow to Lake Okeechobee amounts to 2.39 billion cubic meters per year. 31 3. In Hartwell's USGS report he states that average evaporation from Lake Okeechobee is 2.00 billion cubic meters per year. 4. In USGS Surface Water Records for Florida extreme flows are given for the regulation canals which total 10.0 billion cubic meters per year. 5. Hartwell reports long-term average flow of water out of the Miami, Hillsboro, North New River, and West Palm Beach Canals combined average (1940-63) 360,000 cubic meters per year. 6. USGS Surface Water Records for Florida report a maximum short term backpumping rate of 9.11 cubic meters per year. 7. According to Parker and others (1955) average rainfall on SE Florida is 53 inches per year. If this is prorated over 2 the area of Dade, Broward, and Palm Beach counties (5907 mile) this equals 20.6 billion cubic meters per year. 8. Parker and others (1955) reported average evapotranspiration of 45 inches annually in SE Florida which converts to 17.5 billion cubic meters when prorated over the entire area of the three counties. 9. Hartwell reported other runoff from the counties to be about 2.59 billion cubic meters per year. 10. From U. S. Army Corps of Engineers stage-rating curve for the lake the volume of water at 14.75 feet above MSL is 4.77 cubic meters. 11. Prorating four inches of water over the county area this totals 1.56 billion cubic meters.

33 synchronous rainfall and runoff are shown by a solid line; lake storage that results if runoff lags three months after rainfall is shown by a broken line. Discharge from the lake is increased when water levels reach 14.0 feet MSL in the summer and 15.5 feet MSL in the winter as part of flood protection. Under normal rainfall-runoff conditions with asynchronous timing (before channelization) a greater percentage of water entered the lake in winter months when allowable levels were higher. When rainfall and runoff enter the lake in synchrony (a highly channelized condition) allowable levels are low and some of the total inflow is discharged to the sea and is unavailable to raise lake stages in winter months. Figure 11 suggests that about 200,000 acre-feet of storage is lost yearly when runoff is synchronous with rainfall. More water is stored in the lake under drought conditions when runoff lags: rain. The flood hydrograph shows higher lake stages when rain and runoff enter the lake at the same time because the total inflow reaches the regulatory discharge capacity. Another analysis of the effects of channelization ,'8 it affects timing of inflow and lake storage is given in Tables 3, 4, and 5. Table 3 is long term (1931-65) rainfall and evaporation on the lake. Table 4 lists average monthly surface inflow during 1951-60 (A), normalized monthly surface inflow during 1965-74 (B), net atmospheric water (e), total inflow to the lake for 1951-60 (A+e), and total inflow to the lake to the lake for 1965-74 (B+C). In Table 5 monthly discharge is calculated to equal the amounts of water needed to releRsecl for closest maintenance of regulation regime.

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Figure 11. Results of simulating water model shown in Fig. 10. Simulations of lake storage in which rainfall and runoff were synchronous is shown by solid line. Lake storage affected by runoff lagging three months after rainfall is shown by broken lines. These are two year hydrographs for normal (a), drought (b), and flood (c) conditions.

36 Table 4 shows four months of net loss of lake water because of changes in timing of runoff during 1965-74 and no months of net water loss during the earlier period. Table 5 shows more water (248,000 acrefeet) must be released in the summer months of the 1965-74 period because of higher total summer runoff. Table shows lower stages at the end of the year in the 1965-74 period. Agricultural water requirements would affect this analysis by making ending storage even lower for the 1965-74 period. Phosphorus Systems in Lake Okeechobee A model of the phosphorus system in the lake was developed to investigate response times and long term changes in phosphorus levels as they are affected by various nutrient input rates and sediment-water exchange rates. The model of phosphorus in Lake Okeechobee includes allochthonous sources and sinks, water-marsh exchanges and water sediment exchanges. Figure 12 is a model of phosphorus relationships in Lake Okeechobee including mathematical terms for each source, storage, and flow. The solid lines represent relationships explicitly programmed on computer and the broken lines represent functions implicit in the program and are held constant for each simulation run. Evaluation of phosphorus interactions on a unit area basis is presented in Fig. 13; the values for inflow and outflow are from Davis and Marshall (1975). In Table 2 are values and calculation of parameters used to simulate this model. Dynamo programming equations and other information on this model also are presented in Appendix B. In Figs. 14, 15, and 16 are results of simulations of this phosphorus model. In each figure are four separate simulations in which model conditions were changed. Each simulation is 25 years of a particular

TABLE 6. (Continued) 1. Davis and Marshall (1975) reported total phosphorus concentrations in lake average 0.054 g P/m 2 Assuming an average depth of 8 feet at a stage of 14 feet MSL, the standing crop equals 0.132 g P/m 2 2. The author made biomass clippings of various marsh communities in Lake Okeechobee near Clewiston during August 1974. A quarter m 2 of marsh was cut, dried at 70C for five days and weighed; the dry weights were multiplied by the factor 0.4 to convert to grams carbon. The results were: bullrush-680 g C/m 2 cattail-380 45 g C/m 2 spikerush-150 g C/m 2 peppergrass-90 g C/m 2 pond lilly-IOO g C/m2 (estimated). The weights were multiplied by the areas of these communities given by Ager (1974) to get a total of 65.2 X 109 g C in marsh plants. This equals 37.6 g C/m2 when prorated over the lake area of 1.736 billion m 2 Assuming a C:P ratio of 100:1 in these plants, the average phosphorus standing crop is 0.376 g P/m 2 Phosphorus in marsh soils can be estimated by assuming a bulk density of 1 g dry wt/cm3 which equals 104 g dry wt/m2/cm depth. Assuming depth of sediments in marsh is 5 cm deep and concentration is 0.27 x 10-3 g pig dry wt of sediment (Joyner, 1974). The standing crop of phosphorus in marsh soils is (5 x 104 x 0.27 x 10-3 =) 13.5 g P/m 2 to 5 em. Prorate this value overmarsh sediment area (0.276 billion m 2 ) and the whole lake area (1.736 billion m 2 ) which equals 2.15 g P/m 2 The phosphorus in the marsh plants and soils is 2.53 g P/m 2 3. Total phosphorus stored in sediments in the open water zone can be estimated from calculation in note 2 and prorating the standing crop to 5 em (13.5 g P/m 2 ) over the area of open water zone dividing by the area of the whole lake. This equals 11.35 g P/m2 of lake. 4. In Table 6 is the estimation of the phosphorus load delivered to the lake in 1952 (Odum, 1953). Phosphorus load was estimated to be 0.102 g P/m 2/year. 5. As estimated by Joyner (1974) and correcting for time and area of the lake this amounts to 0.389 g P/m 2/year. 6. Davis and Marshall (1975) estimated an areal loading rate of 0.33 g P/m 2/year during 1973-74. 7. Area based loading rates from Lake Apopka was estimated from Sheffield and Kuhrt (1970) by totaling the Zellwood drainage district load of 800,000 lbs o-P04 or 0.9 g P/m 2/year, Winter Garden area load of 245 lbs or 0.11 g P/m 2/year, and rain influx of about 0.082 g P/m 2/year. This totals 1.1 g P/m 2/year. 8. Using the model Kirchner and Dillon (1975) based on areal loading rates of water to a lake, it predicts the retention of about 80% of the phosphorus entering Lake Okeechobee. Twenty percent leaves which is (0.20 x 0.102 g P/m 2/year) 0.0204 g P/m 2/year.

1.76 gC/m 2/day or 642 gC/m 2/year. Assuming the C:P ratio was 100:1 in this material, the amount of phosphorus sedimented equals 6.42 gP/m 2/year. The production and respiration values are from Davis and Marshall (1975). The level of higher consumption is estinlated from data on fish, zooplankton, and bottom animal populations by Joyner (1974), Ager (1974), and Davis and Marshall (1975). 16. Assuming sedimentation and recycle are equal, the recycle of phosphorus was evaluated at 6.42 gP/m 2/year. If this is equivalent to 642 gC/m 2/year or 0.18 g02/m2/hr, it places sediment respiration in the range normal for Florida lake bottoms (Tom Ballinger, pers. comm.). 47

49 model condition during which marsh phosphorus (M), sediment phosphorus (S), and water column phosphorus (W) were monitored. Figure 14 shows the effect of different phosphorus loading rates on the levels of phosphorus in the lake. Loading rates calculated from studies by Odum (1953), Joyner (1974), Davis and Marshall (1975), Sheffield and Kuhrt (1970) affect the results of simulations in Fig. l4a, l4b, l4c, and l4d, respectively. The phosphorus loads based on Odum's study represent more natural conditions, loads based on Joyner's study and Davis' study represent present levels, and loads based on Sheffield's work with Lake Apopka represent very high load rates. Figure 14 shows that the rate of phosphorus build up increases with load rate. After 25 years water column phosphorus levels show increases of 12%, 49%, 58%, and 147% with each increasing net load to the lake in Figs. l4a-14d. This equals rates of increase of water column phosphorus of 0.6 parts per billion (ppb)/year, 2.6 ppb/year, 3.1 ppb/year, and 7.8 ppb/year, respectively for Figs. l4a-14d. Joyner (1974) measured an average 47 ppb total phosphorus from 1969-72 and Davis and Marshall (1975) reported an average 54 ppb total phosphorus from January 1973 to May 1974. Odum (1953) reported total phosphorus levels lower than any measured during later studies which averaged 13 ppb. The difference between earliest measurements and latest measurements is 41 ppm r"-0f": water column phosphorus. The marshes in these simulations remained the same because of the small rates of exchange between water column phosphorus and marsh phosphorus. Figure l5a shows the same simulations as l4c which is the lake under the influence of 25 years of phosphorus loads measured by Davis. All of the following simulations in Fig. 15 and Fig. 16 have Davis' loading rates but have variations of other parts of the model. Figure l5b shows the

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Figure 14. Results of simulating phosphorus model shown in Figs. 12 and 13 for 25 years of four different inflow rates. In order of increasing net annual delivery to the lake were: (a) inflows of 1952 (Odum, 1953); (b) inflows of 1969 (Joyner, 1974); (c) inflows of 1974 (Davis and Marshall, 1975); and (d) inflows of area-corrected flows to a hypereutrophic lake, Lake Apopka (Sheffield and Kuhrt, 1970).

52 effect of phosphorus losses by aerosol formation which equals rainfall loads. In this simulation water column phosphorus increases 41% in 25 years. Water column phosphorus rises similarly in Fig. l5c when marsh uptake is increased five times. Figure 15d shows the effect of marshes dying off and releasing phosphorus to the water column. This accelerates rises in water column phosphorus to 79% over 25 years; sediment phosphorus shows similar rise. Figure l6a shows phosphorus levels from Davis loading rates. Figure l6b shows the effect of desorption of sediment phosphorus under anaerobic or low pH conditions. The water column phosphorus levels rise suddenly in the first five years up to an 83% increase over the period simulated. Figure l6c shm
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57 phosphorus, dissolved oxygen, total organic carbon, sediments and consumers are represented in the various assimilation, production and recycle pathways. The mathematical relationships presented in Fig. 17 are replaced by quantified values for storages and flows in Fig. 18. In Table 8 is a list and description of these model parameters which are annotated. Since flow of allochthonous organic carbon loads from tributaries to the lake can account for present saturation deficits of dissolved oxygen observed in the lake, the decomposition of the marsh's organic pool may be separate from that in the limnetic zone. Low current velocities and low turbidity measurements just lakeward of the marsh support this assumption. The simulation results in Fig. 19 show diurnal changes in the lake parameters for five days of average conditions. Dissolved oxygen levels reached equilibrium with the carbon loads in five days, varying between 7.0 and 7.5 ppm. Total dissolved phosphorus varied between 32 and 38 parts per billion with highest values at sunrise after five days of average conditions. The parameters of sediment, and consumers held steady, and the water column carbon increased by about 0.1 ppm/day. The graphs in Fig. 20 show the effects of low diffusion on the lake's oxygen levels. In five days the oxygen levels dropped to 5.3 ppm and water column carbon and sediment carbon increased 33% and 10%, respectively. The increases in standing crops of carbon was due to decreased oxidation. The graph in Fig. 21 representing a 100-fold increase in phosphorus loading to the lake, shows phosphorus levels rising to 0.1 mg/l in five days. These high levels of phosphorus increase production of oxygen to to levels of super-saturation (120-140%). The high productivities cause greater diurnal variation of water column carbon and dissolved oxygen.

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Figure 17. Model and mathematical equivalents of daily metabolism in Lake Okeechobee. The model in broken lines shows the larger system given in Fig. 2 from which this model was simplified.

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...., == x .x 59

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Figure 18. Model of metabolic system in limnetic zone of Lake Okeechobee. Values for flows and storages are included.

TABLE 8. (Continued) 1. Joyner (1974) reported 0.389 gP/m 2/year loading to Lake Okeechobee which equals 0.0011 gP/mZ/day. Z. Joyner (1974 reported 0.114 gP/mZ/year leaving by surface outflows which equals 0.0003 gP/m 2/day. 64 3. Davis and Marshall (1975) reported an average daily primary production of 2.1 gC/mZ/day. Assuming for every 100 grams of carbon fixed there is 1 gram of phosphorus fixed, and half of the phosphorus fixed is supplied internally from phosphorus storages in the planktons. Daily assimilation of phosphorus equals 0.011 gP/m 2/day. This needs to be measured in lab or field experiments. 4. Davis and Marshall (1975) reported respiration was 10% of gross primary production (GPP) which they estimated to be 2.1 gC GPP/m 2/day. In percent of the GPP is 0.21 gC/m 2/day of plankton respiration. If the consumer population which has a standing crop of 18.6 gC/m 2 (see note Z3), and assuming 90% of the consumer's food requirement comes from the water column, this represents a flow of 0.18 gC/m 2/day from water column carbon to consumers. The recycle of phosphorus from plankton respiration (0.21 gC/mZ/day) and consumer activities (0.18 gC/m 2/day) assuming a C:P ratio of 100:1 and a release of one-half this amount (0.18 + 0.Zl)(0.01)(0.5) equals O.OOZ gP/m 2/day. 5. The recycle of phosphorus from sediments depends on the rate of respiration of organic carbon in the sediments. The respiration of organic carbon in sediments is assumed to equal the total fixed (Z.l gC/m 2/day) plus allochthonous sources (0.07 gC/mZ/day) minus respiration of plankton (O.Zl gC/mZ/day) and minus the amount consumed by the higher trophic levels (0.20 gC/m 2/day). The calculated sediment respiration (2.1 + 0.07 0.21 0.20 gC/m 2/day) equals 1.76 gC/m 2/day. Recycle of phosphorus equals one-half of the phosphorus equivalent of carbon respiration (0.5 x 0.01 x 1.76 = 0.0088) minus that which is sorbed into the sediments. Sorption rates were calculated (from phosphorus model) to be 0.365 gP/mZ/year or 0.001 gP/mZ/day. Phosphorus recycle (0.0088 0.0010 gP/m 2/day) equals 0.0078 gP/mZ/day. In simulation this value was rounded off to 0.008 gP/mZ/day. 6. Diffusion of oxygen is assumed to equal moderate levels quoted by Odum (1957) for lakes: 1 g02/mZ/day at 100% saturation deficit. 7. Oxygen production equals the oxygen equivalent of 2.1 gC/mZ/day of GPP: (Z.l gC/m 2/day x Z.5 gOZ/gC=) 5.25 gOz/mZ/day. 8. Respiration in the water column (see note 4) equals 0.39 gC/m2/day; the oxygen equivalent of this is 0.98 gOz/mZ/day. 9. Respiration in sediments is 1.76 gC/mZ/day plus consumption of sediments (10% of the food requirement of consumers) which is O.OZ gC/mZ/day. The oxygen equivalent of this (1.78 gC/mZ/day x Z.5 g02/gC) is 4.45 gOz/mZ/day.

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10. Davis and Marshall (1975) reported 2.1 gC/m 2/day for annual lake wide average. 65 11. Allochthonous organic carbon inflows were estimated from Brezonik's (1975) estimation of total organic carbon (TOC) in back pumpage water (100 mg/) times average backpumpage flow (350,000 acre feet/year) and from Joyner's (1974) measurements of TOC in other tribu taries which average 20 mg TOC/ and flow at 2,000,000 acre-feet/year. The inflow of carbon is 0.15 g TOC/m2/day. Outflows of carbon (2,000,000 acre-feet/year at 20 mg/) equals 0.08 g TOC/m2/day. The net organic carbon delivered to the lake is 0.07 g TOC/m2/day. 12. The rate of sedimentation equals the amount of carbon fixed (2.1 gC/m 2/day) minus carbon respired and consumed in the water column (0.39 gC/m 2/day) plus allochthonous carbon (0.07 gC/m 2/day). This equals 0.78 gC/m2/day of net sedimentation. An arbitrary rate of suspension of carbon was set at 48 gC/m 2/day, which represents the replacement of the standing crop of water carbon once a day. The gross sedimentation rate is 1.78 gC/m 2/day plus 48 gC/m2/day or 49.78 gC/m 2/day. 13. The gross rate of suspension of organic carbon was assumed to replace water storages once a day: 48 gC/m 2/day. 14. Organic carbon respired and consumed from the water column (see note 4) is 0.39 gC/m 2/day. 15. Organic carbon respired from sediments equals 10% of the food needs of the consumer population (0.02 gC/m 2/day) plus 1.76 gC/m 2/day of sediment respiration: 1.78 gC/m 2/day. 16. Consumption of water column carbon by consumers was estimated to be 90% of total food consumed (see note 23): 0.18 gC/m 2/day. 17. Consumption of sediment carbon by consumers was estimated to be 10% of total food consumed (see note 23): 0.02 gC/m 2/day. 18. Assuming consumers are in steady state their production (0.20 gC/m 2/day) equals their respiration and mortality (0.20 gC/m 2/day). 19. Standing crop of total dissolved phosphorus was estimated by averaging Joyner's (1974) phosphorus determination of filtered samples: 0.012 g dissolved and colloidal phosphorus/m3 and 0.029 g P(dissolved)/m2 at an average depth of 8 feet. 20. Saturation of 02 in freshwater at 25C and 760 mm of pressure is 8.4 ppm. At an average depth of 8 feet this (8.4 g/m3 x 2.44m) equals 20.5 g02/m2. The initial condition was set slightly below this. 21. Total organic carbon measurements by Joyner (1974) and Brezonik (1975) average 20 mg TOC/. Over a depth of 8 feet this equals 48.8 gC/m2. The initial condition in simulation was set slightly lower. 22. At a bulk density 1 cm3/g, one m 2 of sediment which is 10% organic matter (Burton et al., 1975) and one centimeter deep would con tain 500 gC/m2.

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66 23, Consumer biomass was estimated to be 7.0 gC/m 2 of fish (Ager, 1974), 1.0 gC/m 2 of zooplankton (typical level of zooplankton for eutrophic lakes; Nordlie, personal communication), and 10.6 gC/m 2 of benthic invertebrates (numbers of invertebrates Joyner, 1974; Davis and Marshal, 1975). The total is 18.6 gC/m2. Benthic invertebrate net production is 73.6 x 109 gC/year, zooplankton net production is 30.9 x 109 gC/year, fish net production is 17.1 x 109 gC/year. The total net production was calculated to be 0.20 gC/m 2/day. The net production of zooplankton and benthic fauna was estimated from Prosser and Brown (1961), Hall (1964), and Waters (1969). The net production of fish was estimated from Gerking (1962), using standing stocks from Ager (1974). The biomass of benthic fauna was calculated by estimating individual weights per organism and multiplying them by numbers of organisms estimated by Joyner (1974) and Davis and Marshall (1975).

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Figure 19. Simulation of metabolism model shown in Fig. 18 for five days of average conditions. Shown are daily changes in oxygen, phosphorus, sunlight, carbon, and consumers.

2 Primary production is about 5 g Clm Iday. 73 The graph in Fig. 22 represents five days of average lake conditions with a ten-fold increase in organic carbon load. The standing stocks of carbon in the sediment and water column rise similarly. to conditions encountered in the simulation of low diffusion. The oxygen sags to normal (80% saturation) levels in five days and reaches lm,er levels of about 60% saturation in steady state. Spatial Patterns in Lake Okeechobee The EPA Storm Water Receiving Model was modified to simulate currents and spatial patterns in Lake Okeechobee under conditions of high runoff, backpumping and strong south winds. Results of the model after five days of simulation is given in Fig. 25. The highest flow rates are in the lim-netic zone of the lake bet,,,een the nodes 5-11-19. The high flow rates from node 1 to node 5 suggest that Taylor Creek water can be advected into the lake from currents induced by the Kissimmee River. The low flow rates in the marsh areas (0.000 to 0.040 feet per second) suggest that transport is low in the marsh and the limnetic and marsh zone are hydraulically distinct. The simulation also indicates lower flow rates in the area of backpumpage at the south end of the lake. This would imply that the hydraulics in the South Bay area tend to restrict the spread of backpumpage water into the lake. These hydraulic characteristics of the lake are supported by obser-vat ions made by FCD and NASA (Marshall, pers. COllUll.).

Figure 24. Results of simulation of spatial model after five days of high flow from tributaries and pumps. Arrows on the channels (lines) show direction of flow and numbers are flow rates in feet per second.

DISCUSSION Simulations of models suggest that Lake Okeechobee has changing water regimes and nutrient inflows due to development in watersheds around the lake. Further development in the basin could compound problems in an already eutrophic lake. Channelization and Water Levels 80 Channelization of tributaries to Lake Okeechobee has accelerated runoff so that rainy season flow accounts for greater percentages of total annual inflow than under prechannelized conditions (Fig. 8). Although flooding potential is less upstream in these channelized basins, potential flooding downstream is increased. Moderately high rainfall in 1969 caused record flow rates (21,700 cfs) in the Kissimmee River channel which seriously damaged the S-65E control structure; hurricane rains will help determine the efficacy of this flood control system in the future. Changes in timing of inflow to Lake Okeechobee causes lake stages to rise more quickly and sooner during the rainy season. In order to maintain safe storage capacities in the lake some of the inflow is discharged through the St. Lucie and Caloosahatchee canals. Generally, high rates of inflow require high rates of outflow through regulation to control lake stages. Consequently, accelerated runoff in channelized tributaries may increase runoff and flood stress in the regulation canals (Tables 3, 4, and 5). Accelerated runoff may also cause decreased average annual storage in the lake if summer discharges are increased to

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81 keep stages low and fall and winter inflows deliver insufficient water to raise and maintain higher lake stages (Fig. 8; Tables 3, 4, and 5). Increased tributary flow variability (Heaney and Huber, 1974) may cause increased variability in lake stages (Fig. 8). Although greater ranges in storage may be desireable for managed drawdowns in Lake Okeechobee, this may be undesireable for water supply and navigation purposes. Decreased base flow (Fig. 8) and total flow (Fig. 9) from channelized basins cause lake stages to be lower in the dry season (Fig. 8) and during droughts (Fig. 11). Channelization and drainage of river basins around Lake Okeechobee has reduced water levels in these basins to the extent that marshes and pastures become parched in dry periods and are less useful for grazing (Gatewood, 1975). Lowered water tables may also cause oxidation of organic soils (Hortenstine and Forbes, 1972) and release of nutrients which are washed to the lake during rainy periods. This decreases the natural fertility of the land and increases the eutrophy of lakes. Formerly surface waters percolated more into the ground reaching runoff streams later and apparently in greater quantity and quality. Water Levels and the Lake Ecosystems Water levels in Lake Okeechobee have a great effect on areas of water and marsh exposed. At a lake stage of 10.5 feet MSL there are 326,000 acres of water surface area and at 16.0 feet MSL 454,000 acres are covered by water. The lake's emergent marshes have developed between the 11.0 and 14.0 feet MSL contours (Ager, 1974) with average water levels at about 14.0 feet MSL. The area between the 11 and 14 foot contours is about 97,000 acres of which about half is emergent marsh

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82 and half is ponds and open water. If emergent marshes maintain themselves at zero to three feet below average water levels in the lake, a two-foot increase in average lake stage would put emergent marshes between the 13.0 and 16.0 feet MSL contours. The area between these contours in 57,000 acres, or 40,000 acres less potential area for emergent marsh. However, marshes that grow in deeper water may increase in area at higher regulation levels. Bullrush and submerged communities now occupy the area between the 8.0 and 12.0 feet MSL (87,000 acres), which is two to six feet below mean lake stage. If these communities were to grow between the 10.0 and 14.0 feet MSL contours in response to a two foot rise in average lake stage, these communities could cover 120,000 acres, or an additional 33,000 acres over the same marshes at lower stages. Since wading birds depend heavily on the shallow (seasonal) marshes in Lake Okeechobee when other feeding areas in south Florida have dried up (Browder, 1974), an increase in lake stage regulation levels and subsequent decrease in area of seasonal marshes could reduce wading bird populations. Ager (1974) reported highest levels of fish and game fish values in the outer most marsh zones and if higher stage levels increase the area of these zones, proposed regulation levels could increase fish populations in littoral areas. Furthermore, greater ranges in stages at present and proposed levels could maintain or increase the areal extent of seasonal and deep marshes by allowing colonization at lower stages. This principle was used to increase the extent of littoral zone in a lake north of Okeechobee (Holcomb, 1972). Gary Pesnell of the Central and Southern Florida Flood Control District is investigating in greater detail the relationship of water levels to marsh growth.

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83 The limnetic ecosystem is also affected by lake stage variations. Volume changes in the lake dilutes or concentrates nutrients, and depth changes affect turbidity in the lake. Joyner (1974) noted that secchi disk readings were lowest and dissolved solids were highest when lake stages were low. Changes in lake stage affect the area of limnetic zone in production. Lake depth also affects the degree to which the water column and bottom sediments are mixed. Price (1947) developed a regressio.n model which predicted maximum equilibrium depth from average lake width. His model was developed for enclosed tidal basins along the Texas-Louisiana Gulf coast but was not dependent on basin origin or variety of soft sediments within the basin. Applied to Lake Okeechobee, this model predicts a maximum equilibrium depth of 16.5 feet. Deepest areas'in Lake Okeechobee average 15.0 feet below long term average lake levels (14.0 feet MSL), and the model suggests the lake is near equilibrium and is tending to scour deeper. If lake stages are raised, the tendency for scouring will be less in the deeper areas of the lake. Higher lake stages and reduced sediment scouring may reduce oxygen demands on the water column, cause sediment to accumulate, lower redox potentials, and increase acidity in the sediments allowing for the release of phosphorus to the water (Figs. l6b and l6d). However, large lakes generally have thorough mixing to depths of greater than 10 m (Hutchinson, 1957). Phosphorus Loading and Eutrophication Total nitrogen and phosphorus ratios in Lake Okeechobee, which average about 35:1 by weight, suggest phosphorus is the limiting nutrient. Phosphorus limitation also is suggested from the higher productivities

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84 measured at the north end of the lake where phosphorus loading is greatest. Lake Apopka, which has total phosphorus levels about three times those in Lake Okeechobee, has productivities approximately three times those measured in Lake Okeechobee (Davis and Marshall, 1975). Inflow rates of phosphorus to Lake Okeechobee are high, 0.3 to 0.4 gm P/m 2/year. Of the phosphorus entering the lake from tributaries and rain only 10% is measured leaving the lake from surface outflows. Since these large inflows of phosphorus are not accounted for in the water column, some must be lost to air, marshes or sediments. Although most phosphorus in rainfall comes from bubble collapse and aerosol formation at sea, little is known about this phosphorus transport mechanism in fresh waters. Baylor and others (1962) showed no loss of phosphates from aeration of solutions made up of either distilled water or artificial sea water, whereas aerated seawater showed phosphorus losses. Measurements should be made on Lake Okeechobee to determine how significant this outflow may be. If large amounts of phosphorus are assimilated and stored in marsh macrophytes, increases in standing crop or concentrations could be measured. However, the hydraulic character of the lake causes most of the inflowing phosphorus to enter directly into the limnetic zone (see Table 6, note 12). By the time a parcel of water reaches the marshes it already may be reduced in phosphorus content. Phosphorus can be lost to sediments by coprecipitation, adsorption and organic sedimentation. Based on theoretical data by Green and Holmes (1948), solubility of phosphorus is dependent on the pH and calcium and pH in Odum (1953) and Barrett (1952). In Lake Okeechobee the pH averages about 8.4 and calcium concentrations averages 40-50 ppm,

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85 resulting in an ortho-phosphate solubility equilibria of about 0.020 ppm. Joyner (1974) found the lake to average between 0.054 and 0.029 ppm total ortho-phosphate, considerably higher than equilibria. The supersaturated tendency favors precipitation of phosphate minerals by mollusc shell formation and other processes. Adsorption of phosphorus in sediments is a faster process than coprecipitation and could account for a large proportion of the phos-phorus sink. Harter (1968) and Kamp-Nielsen (1974) reported aerobic lake muds adsorbed most of the dissolved or tho-phosphate in overlying waters within a few hours. Since Lake Okeechobee sediments are almost always aerobic and basic (Burton et al., 1975), rapid sorption of phos-phorus is likely. Biological uptake and sedimentation of organic matter may also account for large losses of phosphorus loads. If volume of organic sediments is any measure of this process, then the some 300 million cubic meters of organic sediment in Lake Okeechobee might suggest it is important (Gleason, pers. comm.). 2 Assuming the loss of 0.29 g P/m /yr (David and Marshall, 1975), a concentration of 0.27 g P/kg dry weight, then organic sedimentation could account for 1 mm of dry sediment/year and could represent up to 0.5 cm wet sediment/year. Surveys of sed i-ments would help determine its relative importance as a phosphorus sink. Recycle of phosphorus from the is dependent on the redox potential, pH, degree of mixing, and biological activity in the sub-strate. Anaerobic and acid sediments tend to release phosphate. During respiration the process of oxygen consumption and carbon dioxide pro-duction lowers both pH and oxygen content of the sediments and overlying waters. If the sediments are alternately settled and reworked, phosphorus

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86 solubilized in quiescent periods is released when sediments are suspended (Burton et al., 1975). The comparison of flow magnitudes in Fig. 13 suggests that rates of sedimentation, sorption and recycle are key factors determining the immediate levels of water column phosphorus. Loading rates may be many times less than rates of interchange with the sediments and serve to elevate phosphorus levels over long periods. According to this model, water-sediment phosphorus interchanges alone replace sediment stocks every 1.6 years and replace water stocks every 7 days, whereas allochthonous flows replace these stocks in about 34 years and 0.5 years, respectively. Consequently, variations in sediment-water inter-changes may alter short term phosphorus levels, whereas loading rates change phosphorus levels over long periods. Whether sediment-water phosphorus interactions are in steady state with loading rates is of utmost importance in determining eutrophication trends. If recycle increases or sorption decreases with continued load-ing rates, water column phosphorus will rise to higher levels. Burton and others (1975) studied sorption rates of sediments in the Kissimmee River and Lake Okeechobee and found that these aerobic sediments had 2 maximum sorption of 0.01 g P o-P04/m over a few hours after which no sorption occurred. In the case of the simulation presented in Fig. 14, the water column phosphorus rises continually for the period simulated, recycle being a linear function of sediment phosphorus levels. For present loading rates, these simulations show 2% per year increases in water phosphorus, which correlates with percent increases between average levels in 1969 (0.050 mg/l) and average levels in 1973-74 (0.054 mg/l).

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Although this is not validation of the model or future trends because of large variances in yearly averages and short time periods, it suggests further long-term increases in phosphorus levels could occur. 87 These simulations also show relationships between loading rates and rates of phosphorus increase. Low loading rates (Fig. l4a) showed gradual increases in water phosphorus over long periods, increases which suggest the slow, natural rates of increase in phosphorus and eutrophication. Rates of increase of water phosphorus in Fig. l4d (6% increase per year) shows the process of an eutrophic lake becoming hyper-eutrophic in a short time. Simulations in Fig. 16 show effects of constant recycle rates, higher recycle rates, and anaerobic release of phosphorus on nutrient levels in the water column. Constant recycle rate keeps water column levels in steady state. Accelerated recycle rates increase the rate of build-up of water column phosphorus. The change of aerobic sediments to anaerobic sediments causes much more phosphorus to be released from sediments. Kap-Nielsen (1974) reported anaerobic desorption rates 10 times greater than aerobic sorption rates. It is difficult to determine whether water phosphorus levels and production will increase if present loading rates continue. It is unlikely that phosphorus levels will increase drastically in the lake water for the next few years if loading and mixing characteristics remain as they are presently. However, marked increases in loading rates or reductions in the pH and redox character of the lake would quickly elevate water phosphorus levels. If further eutrophy is not desired, care should be taken not to overload Lake Okeechobee with nutrients; evidence from other shallow, highly eutrophic lakes suggests that

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88 recovery is slow even after diversion of much of the nutrient load (Mathieson, 1971; Andersson, 1973). Furthermore, Barrett (1952) postulated lakes with greatest potential of elevated water phosphorus levels from fertilization were lakes with no stratification, high marl and organic matter content in sediments, and moderate alkalinities; a good description of Lake Okeechobee. Chemical Loadings and the Metabolic System Results of simulating the metabolic system indicate that oxygen levels in the lake as a whole would not fall to critical levels on the short term under anticipated conditions of carbon or phosphorus loading. The simulation, in which diffusion was low and organic loading was high, suggested depressed dissolved oxygen levels (5 ppm). However, it is unlikely that low diffusion conditions (still waters) could last very long in Lake Okeechobee. These results show that under most expected conditions, lake-wide fish-kills would be unlikely. Although lake-wide oxygen depression is not serious, some areas are presently critical. Many fish kills have been noted in the backpumpage area and in the rim canal because of low oxygen levels (see Appendix C). Occasional high BOD loadings from other tributaries also can depress oxygen levels enough to kill fish. Accelerated production caused by high nutrient loads (Fig. 21) in combination with high respiration from organic matter produced can increase the diurnal range of oxygen levels. Oxygen minima in the early morning during very productive periods can also cause fish-kills. Highest nutrient loading at the north end of the lake caused frequent blooms there (Appendix C).

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89 Simulation of spatial patterns in Lake Okeechobee indicate that slow movement of water in the area of backpumpage allows time for high BOD material to reduce oxygen levels in that area. The hydraulic characteristics of the rim canal were not included in the spatial model but would exert a large effect on it. Commercial Fishing and Eutrophication According to Ager (1972) commercial fish harvest in Lake Okeechobee 2 from 1968-70 amounted to 2.3 lbs/acre/year or 0.26 g wet wt/m /year. In terms of phosphorus removed from the lake this harvest amounts to 0.0014 g P/m2/year if one assumes 0.278 g dry wt/g wet wt (Gerking, 1962) and 2% phosphorus in dry fish matter. Present levels of harvest do not appreciably affect the phosphorus budget of the lake. However, Ager stated that commercial fish harvest potential in Lake Okeechobee 2 could be 76 lbs/acre/yr which would remove about 0.046 g P/m /year based on the same assumptions of the previous calculation. A harvest of this magnitude could affect the phosphorus budget of the lake, could reduce rates of eutrophication, and would stimulate considerably the area's economy_ The ramifications of such ecosystem management should be investigated and tested. Eutrophy in Lake Okeechobee Development and water management practices in basins surrounding the lake increased inflows of nutrients and organic matter increasing production and eutrophication in Lake Okeechobee. Although it is not certain whether present nutrient inflows will cause continued increases in nutrient levels, it is certain that increased nutrient loads could

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90 make the lake hyper-eutrophic. Changes in the alkaline, aerobic conditions in the lake could induce more eutrophic conditions. Because of storages in sediment and marsh, about 10 years are required to increase or decrease phosphorus levels in response to changed inflow.

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APPENDIX A. Supplemental Information for Water System Model as given in Fig. 10. Summary Tables for Hand Calculated Water Budgets as Given in Fig. 7.

TABLE 12. List of Biological Events in Lake Okeechobee Algal Blooms Fish August 1967 August 1968 April 1970 May 1970 June, July 1970 February 1971 May 1971 June 1971 Kills December 1967 June 1968 July 1968 September 1971 July 1972 September and October 1972 June 1973 Bird Kills June and July 1971 Blue-green in north end of lake Blue-green widespread in north end of lake Blue-green in C-38, SE, S-65D, S-65C and in lake around river mouth Blue-green in rim canal at Little Saratoga Blue-green in north end of lake Blue-green in northwest area of lake Blue-green in north end of lake Blue-green in C-38, S-G5D and S-65C 10,000 fish killed in dm canal at S-2 lack of dissolved 10,000 fish killed in rin canal at S-2 lack of dissolved oXYGen 10,000 fish killed in rim canal at pump station 127, lack of dissolved oxygen 2,000 fish killed in rim canal at S-2 lack of dissolved oxygen 5,000 fish killed in Fisheating Creek and Tree Dike Canal at Lakeport, lack of dissolved oxygen 500,000 to 1,000,000 white catfish killed due to unknown cause 1,500 fish killed in Taylor Creek, lack of dissolved oxygen 19 species, 398 birds, killed in Fisheating Creek Bay by Clostridium perfringens 107

BIOGRAPHICAL SKETCH Timothy Laurence Gayle was born February 23, 1952, in Gueydan, Louisiana. He attended public schools in Gueydan and Lake Arthur, Louisiana and was graduated from Goretti High School in June, 1969. He attended University of Southwestern Louisiana, Northwestern State University of Louisiana, and Louisiana State University, receiving a Bachelor of Science degree in botany from L.S.U. in December, 1972. In September, 1973, he entered the Department of Environmental Engineering Sciences at the University of Florida where he is presently a candidate for the Degree of Master of Science. 119